This application is the U.S. national phase of International Application No. PCT/FI98/00796, filed Oct. 14, 1998, which designated the U.S.
The present invention relates to a screen defined in the appended independent claims and to a method of manufacturing a reinforcement for a screen plate.
The term xe2x80x9cscreen platexe2x80x9d in the description and claims means, unless otherwise stated, the screen plate of a drum screen, flat screen or the like. The screen plate according to the invention can be manufactured from a plate by machining or by forming in some other appropriate way screen apertures, such as circular screen holes or elongated screening slots therein. The plate can be provided with grooves, protrusions or the like in order to bring about a desired surface configuration.
The screen plate according to the invention can also be manufactured of screen wires by disposing them side by side so that the gaps between them form screening slots. The term xe2x80x9cscreen wirexe2x80x9d here refers to elongated elements of various cross-sectional forms which, when positioned side by side, form screening slots between themselves.
The drum screens and the flat screens according to the invention are typically used in the pulp and paper industry for cleaning and fractionating fibre suspensions. They can, however, also be used in the food industry, for instance.
It is known technique to reinforce and stiffen screens, screen drums as well as flat screens, so that they will resist the forces and dynamic stresses they are subjected to during operation. The screen drums are most often reinforced by means of rings or bands attached to the surface of the drum. In drum screens of the xe2x80x9coutflowxe2x80x9d type, in which the suspension to be screened is introduced into the interior of the screen drum and the accepted fraction flows through the screen drum from the inside outwards, the reinforcing rings are mounted on the outside of the drum. In the drum screens of the xe2x80x9cinflowxe2x80x9d type, in which the accepted fraction flows in the opposite direction, the reinforcing rings are mounted on the inside of the screen drum.
Common to the reinforcements or the supporting elements of the prior art is that they are made of steel. Manufacturing of steel reinforcements comprises several production and mounting stages. In manufacturing, methods known per se, such as rolling, bending, sawing, turning, milling and grinding are used. In some stage of the production and mounting of the reinforcements and supporting members of steel welding generally has to be used in addition to other methods. Heat is then brought to the welding point, which may easily cause deformations and internal stresses in the screen plate and/or the reinforcement to be produced, which weakens the physical properties of the steel. The deformations can harmfully change the measures of the screening slots and the form and measures of the entire drum. In addition, the burrs that are produced as a result of the welding process cause a tendency to plug the screen as fibres stick on them. Welding is thus to be avoided.
These elements made of steel, when subjected to dynamic load, typically break down through crack growth. The internal stresses and the changes in the physical properties in the so-called heat-affecting zone (HAZ) caused by welding render the elements made of steel liable to crack growth.
It is an object of the present invention to provide an improved screen and an improved method of manufacturing a reinforcement for a screen plate, in which the above mentioned disadvantages are minimized.
It is more specifically an object of the invention to provide a durable and strong screen. The object is thereby to provide a screen drum having a reinforcement ring in which crack growth is avoided.
It is a further object of the invention to provide a method of manufacturing a reinforcement for a screen plate which is simple and which can be easily modified.
The object is thereby also to provide a method of manufacturing a reinforcement wherein the problems caused by high temperature are avoided.
A typical improved screen according to the invention for screening or fractionizing pulp suspensions or the like suspensions of the paper industry comprises a screen plate to the surface of which one or several reinforcements made of fibre-reinforced composite material are attached.
The arrangement according to the invention can be used in screen plates that are conventionally made of a sheet-like preform in which screen apertures are formed by machining or in some other appropriate way. The method according to the invention can also be used in screens in which the screen plate is made of screen wires disposed side by side in such a way that a screening slot is formed between adjacent screen wires.
The arrangement according to the invention can be used in screen plates, which are intended for use as screen drums in drum screens or as flat screen plates in flat screens. In drum screens, preferably annular or rib-like reinforcements are used. In flat screens, preferably reinforcing ribs or the like can be used. The reinforcement is preferably formed by the lamination of fibre rovings, which consist of thousands of fibres. The height of the reinforcing ring or rib is typically between about 5 to 100 mm, preferably 5 to 70 mm. The reinforcing rings can be attached to the screen drum for instance 20 to 100 mm apart from each other.
The reinforcement according to the invention is typically made by the lamination of layers of reinforcing fibres by a matrix material. Preferably, at least one fibre chosen from a group of fibre materials comprising carbon, glass, aramide, boron, aluminium or silicon oxide fibres, or some mixture of these, is used as the reinforcing fibre. The reinforcing fibre can of course consist of some other suitable fibre or the like.
The strength of the reinforcing fibres which are used is typically between 1000 and 5500 N/mm2. Preferably a carbon fibre having a high modulus of elasticity and a strength of 3500 to 5500 N/mm2 is used as the reinforcing material. The modulus of elasticity of the reinforcing fibres which are used has to be preferably  greater than 300 GPa. The reinforcement is preferably made of reinforcement fibre bands, also called rovings, consisting of thousands of reinforcing fibres or filaments or of band-like prepregs made of bands. The reinforcing fibres are positioned in the finished reinforcement substantially parallel with the surface of the screen plate.
As matrix material for connecting the reinforcing fibres, the material best suited for the reinforcing material in question is chosen. The matrix material should preferably have a good long-term strength in wet conditions, sufficient heat resistance at least at 100xc2x0 C., good chemical resistance in a pH range of 2 to 14 and a good dynamic loading strength.
As matrix material preferably some thermoplastic or thermosetting resin is used. Suitable thermoplastic resins are for instance polypropylene, polyamide or acrylonitrile butadiene (ABS) and suitable thermosetting resins are for instance polyester, epoxy or phenolic resins.
The purpose of the matrix material, the resin or the plastic is to transmit the forces between the reinforcing fibres. In the laminate the matrix material prevents mainly the reinforcing fibres from moving in relation to each other. The matrix material has only a small influence on the tensile strength of the laminate in the direction of the fibres, whereas it is of importance as regards the load-carrying capacity of the shear forces between the layers. The purpose of the matrix material is to transmit the shear forces.
The present invention renders it possible to laminate reinforcing rings of rotationally symmetrical screens in one production stage only, in which the reinforcing fibre or the band or rovings made of it is wound between a pair of moulds/mould surfaces around the screen. When hardening, the composite formed of the reinforcing fibre and the matrix material wound between the pair of moulds forms the reinforcing ring. When the composite has hardened, the pair of moulds is removed. The desired height of the ring, i.e. its radial measure, is obtained by winding a desired amount of fibres or a desired amount of layers of bands of reinforcing fibre around the screen. The height of the reinforcing ring is usually about 5 to 50 mm.
The reinforcing ring of the composite material according to the invention can also be made in such a way that it is attached on top of a metal supporting ring or band mounted on the periphery of the screen drum. For the duration of the lamination, a ring forming one half of the pair of moulds is thereby positioned on the screen plate on each side of the above mentioned metal supporting ring. The rings forming the pair of moulds are usually radially higher than the supporting ring, and a space equal to at least the axial width of the supporting ring is left between them. In this way the entire supporting ring can be laminated into a reinforcement formed of composite material. The composite material is fed to this space for instance by winding several layers of bands or rovings of composite material around the screen drum on the supporting ring, i.e. in the space between the walls of the mould. So many turns of band or roving is wound around the screen drum that a reinforcement of desired height is obtained. The walls of the mould and the portion of the screen plate or of the supporting ring on the screen plate or of some other element forming the bottom of the mould determine the form of the reinforcement that is produced. The width of the bottom of the mould, i.e. the distance between the walls from each other, can be larger than the width of the supporting ring or the like on the surface of the screen plate. The bottom of the mould is thereby formed partly by the supporting ring and partly by the surface of the screen plate, and the width of the reinforcement that is produced will be larger than the width of the supporting ring.
The reinforcement, the reinforcing ring or band, can be attached to the surface of the screen plate or to the supporting ring mounted thereon by gluing it with the matrix material. The reinforcement can, on the other hand, be attached to the surface of the screen plate by using the geometry of the screen surface, supporting ring, or the like. The reinforcement can thus be attached to the screen surface for instance by using the hooking effect or the reverse relief. The reverse relief means a xe2x80x98negativexe2x80x99 relief, in other words, that the part is e.g. wedge-shaped in such a way that it is broader at the bottom of the mould than at the mouth. If the reinforcement is produced in a mould or a pair of moulds in which the walls are disposed on both sides of the supporting ring, and in which the distance between the walls is larger than the width of the supporting ring, a reverse relief and a mechanical attachment of the reinforcement to the surface of the screen plate can be brought about by choosing a supporting ring of an appropriate shape. A soft fibre-reinforced composite material can be made to flow tightly around the supporting ring when the reinforcement is produced. When hardened the composite material will be fixed to the supporting ring.
The reinforcement according to the invention can, on the other hand, be attached to other elements on the surface of the screen plate, such as a grooved supporting ring for the screen wires of the wire screen disclosed in the International Publication PCT/FI96/00520. The reinforcing fibre material is thereby wound around the screen drum inside the groove of the supporting ring using the groove in the supporting ring as a mould. Annular extensions of the walls of the groove can additionally be disposed on both sides of the supporting ring, which extensions enlarge the radial height of the mould used to produce the reinforcement and if desired also its width. The protrusions or the like in the groove of the supporting ring form fastening members to which the soft reinforcement can be attached.
The reinforcement according to the invention is preferably made of bands, rovings or other reinforcing fibre material dipped in a matrix liquid and which material can be wound in several layers around the periphery of the screen drum between the wall surfaces of the reinforcement mould disposed on the periphery of the screen drum, and then allowing it to harden chemically. The whole production process can be carried out at room temperature. Some systems require that the resin is cured and hardened at an elevated temperature, typically at a temperature of 80 to 200xc2x0 C.
The reinforcement can, on the other hand, advantageously be made of bands, rovings or the like reinforcing fibre material containing solid matrix material, in which the matrix material is twined, for instance, into the reinforcing fibre. Also this band, roving or the like can be wound between the surfaces forming a reinforcement mould disposed on the periphery of a screen drum several runs around the periphery of the screen drum. The solid matrix material is softened/melted by heating it, for instance, in the winding stage preferably to a temperature of about 100-300xc2x0 C. Thus, the matrix material is preferably melted just before the mould. The fibre-reinforced material is finally allowed to solidify in the reinforcement mould.
The temperature of the screen does not rise significantly in production methods described above, so that the adverse effects of high temperature are avoided by the method according to the invention.